Electronic, color, reflective displays are desirable. Such displays may have a broad range of applications, including electronic paper type applications, for example. It is believed that to achieve high color image quality with such a display, each primary color should be addressable at every image location, for example, at each picture element or pixel.
To fill this need, reflective display technologies typically utilize multiple, stacked color layers. Non-limiting examples of such technologies include: Guest-Host Liquid Crystal Displays (GH-LCDs), stacked cell electrophoretic displays and Cholesteric Liquid Crystal Displays (CLCDs).
GH-LCDs are discussed in “Color Design and Adjustment of Dichroic Dyes for Reflective Three-Layered Guest-Host Liquid Crystal Displays”. Mol. Cryst. Liq. Cryst., Vol. 443, pp. 105-116, 2005. As this article explains, GH-LCDs are believed to be suitable for portable information systems because of their low power consumption, relatively wide viewing angle and high reflectance. Three-layered GH-LCDs with subtractive color mixing of yellow, magenta and cyan have been presented as one technology expected to lead to the development of “full-color” reflective displays.
Referring now to FIG. 1, there is shown a schematic representation of a three-layer GH-LCD 10. GH-LCD 10 utilizes dichroic dyes that are dissolved in negative GH-liquid crystals in vertically stacked cells 2, 4, 6. In the illustrated embodiment, cell 2 may contain magenta dye crystals, cell 4 may contain cyan dye crystals and cell 6 may contain yellow dye crystals. Each of the cells is bounded by glass substrates 20 that support electrodes 30, 40. In the illustrated embodiment, electrodes 30 are transparent and may be composed of indium-tin-oxide (ITO). In the illustrated embodiment, electrode 40 is reflective, and may take the form of a metal electrode.
When a voltage is applied to the cell via the electrodes, the helical structure (on state) is realized and the dichroic dyes absorb a large quantity of light. In the off state, the liquid crystals are aligned vertically to the substrate, such that the dichroic dyes absorb only a small quantity of light. Each cell may be controlled independently to realize color imagery.
Stacked cell electrophoretic displays are discussed in U.S. Pat. No. 6,727,873, entitled “REFLECTIVE ELECTROPHORETIC DISPLAY WITH STACKED COLOR CELLS”. Referring now to FIG. 2, there is shown a schematic representation of an electrophoretic display 50. Display 50 includes three stacked cells 52, 54, 56. Each cell 52, 54, 56 is bounded by windows 60. In the illustrated embodiment, windows 60 are light transmissive, and may be composed of glass. A reflective layer 70, such as a metal coating, is supported by the lower-most window 60. Cells 52, 54, 56 are also bounded by side-electrodes 80. Each cell 52, 54, 56 includes a post-electrode 90 and a colorless suspension fluid containing suspended pigment particles. In the illustrated embodiment, the particles suspended in cell 52 may be yellow, the particles suspended in cell 54 may be cyan and the particles suspended in cell 56 may be magenta in color.
Cells 52, 54, 56 may be switched between collected and dispersed states by appropriately charging the electrodes. In the collected state, the suspended particles approach the side-electrodes 80. In the dispersed state, the suspended particles are dispersed over substantially the entire horizontal area of the cell. Light entering a cell in the collected state passes there-through without substantial visual change. Light entering a cell in the dispersed state interacts with the suspended particles, and thereby undergoes a substantial visual change. Different combinations of collected and dispersed states of stacked cells may be used to provide different colored pixels.
Cholesteric Displays are discussed in U.S. Pat. No. 7,061,559, entitled “STACKED COLOR LIQUID CRYSTAL DISPLAY DEVICE”. Referring now to FIG. 3, there is shown a schematic view of a cholesteric display 100. Display 100 includes four stacked cells 102, 104, 106, 108. Cells 102, 104 and 106 reflect light on the visible spectrum, while cell 108 reflects light in the infrared spectrum. Cell 102 contains chiral nematic liquid crystals having a pitch length that reflects red light. Cell 104 contains chiral nematic liquid crystals having a pitch length that reflects blue light. Cell 106 contains chiral nematic liquid crystals having a pitch length that reflects green light. Liquid crystals in cell 108 reflect infrared light.
Display 100 also includes five substrates 110, 112, 114, 116 and 118, the back (or bottom as shown) of which may be painted a color, or alternatively, a separate color imparting layer/substrate 120 may be used. Each substrate may support one or more ITO electrode, passivation and/or alignment layers. Applying an appropriate voltage to the electrodes allows the textures of the liquid crystals, and their reflectivity to be controlled.
Regardless of the particulars of the configuration used, for a pixelated device having three (CMY absorbing or RGB reflecting) or four (CMYK) monochrome display layers stacked on top of each other, the same number of pixels are typically addressed at each layer. This leads to a complex and costly display due to the multiplicity of associated driving circuitry and pixel structures.